1. Field of the Invention
This invention relates to an objective lens driving device and an optical pickup unit. More particularly, it relates to an objective lens driving device for shifting an objective lens in a direction parallel to its optical axis and/or in a direction normal to the optical axis.
2. Background of the Invention
In a conventional optical disc recording and/or reproducing apparatus, employing an optical disc as a recording medium, an objective lens driving device is provided for moving an objective lens, designed for converging the outgoing light from a light source, such as a semiconductor laser, on the optical disc, in a direction parallel to its optical axis and in a direction normal to the optical axis.
With the objective lens driving device, the objective lens is moved, in dependence upon focusing error and tracking error signals, in a direction parallel to the objective lens, that is in a focusing direction, and in a direction normal to the focusing direction, that is in a tracking direction, for focusing the light beam from the light source by the objective lens to the signal recording surface of the optical disc run in rotation by a disc rotating driving device, and for allowing the light beam to follow a recording track on the signal recording surface of the optical disc.
As an objective lens driving device, employed for the optical disc recording and/or reproducing apparatus, one shown in FIGS. 1 and 2 is in use extensively.
The objective lens driving device has on its one end a bobbin 2 carrying an objective lens 1. The bobbin 2 is supported in a cantilevered fashion on a fixed supporting member 6 mounted on a yoke 5 of a magnetic circuit section 4, using four linear supporting members 3, such as wires.
The bobbin 2, carrying the objective lens 1, has a center through-hole 7 traversing the bobbin 2 in a direction parallel to the optical axis of the objective lens 1. A prismatically-shaped focusing coil 8, with a hollow inside portion, is arranged in the through-hole 7. On an outer lateral side of the focusing coil 8 is arranged a tracking coil 9, consisting of a pair of flat rectangular-shaped coils 9a, 9b, in a side-by-side relation to each other.
On each of the opposing lateral sides of the bobbin 2 is mounted a printed wiring board for relaying to which are electrically connected coil terminals 8a, 9a led out from the focusing coil 8 and the tracking coil 9, respectively. On the printed wiring board 10, an elastic supporting member 3 of an electrically conductive material, functioning as a feed line, has its one end 3a electrically and mechanically connected to a connection pattern 10a provided on the printed wiring board 10 using an electrically conductive adhesive, such as a solder.
A pair of such elastic supporting members 3, supporting the bobbin 2 by respective one ends 3a, are formed on each lateral side of the bobbin 2 parallel to each other and have opposite ends 3b thereof passed through through-holes 11 formed in respective corners of the fixed supporting member 6 mounted on the yoke 5 so as to be supported by the fixed supporting member 6. The ends 3b of the elastic supporting member 3 projected via the through-holes 11 of the elastic supporting member 3 operate as connecting ends electrically connected to a driving control circuit configured for moving the objective lens 1.
The objective lens 1, thus mounted on the bobbin 2, supported in a cantilevered fashion on the fixed supporting member 6 via two elastic supporting members 3 on each side thereof by the fixed supporting member 6, is moved in a direction parallel to the optical axis and in a direction normal to the optical axis, as indicated by arrows F and T in FIG. 1, respectively, with the elastic supporting member 3 operating as deflection portions.
On the yoke 5 carrying the fixed supporting member 6 are formed a pair of upstanding pieces 12, 13 facing each other. On the inner surface of the upstanding piece 12 is mounted a magnet 14 constituting the magnetic circuit section 4.
A fixed supporting member 6, carrying the bobbin 2 via plural elastic supporting members 3, is mounted on the upper surface of the opposite end of the yoke 5 for constituting the objective lens driving device. At this time, the upstanding pieces 12, 13 are introduced into the through-hole 7 in the bobbin 2 on either sides of the focusing coil 8 and the tracking coil 9, as shown in FIG. 1. The focusing coil 8 and the tracking coil 9 are arranged at a position of interlinkage with the magnetic flux emanating from the magnet 14 to the upstanding piece 13.
With the above-described arrangement of the objective lens driving device, if the driving current derived from the focusing error signal is supplied from the driving control circuit via the electrically conductive elastic supporting member 3 to the focusing coil 8, there is produced a driving force which, in conjunction with the magnetic flux of the magnetic circuit section 4, operates to move the bobbin 2 in a direction parallel to the optical axis of the objective lens 1. The bobbin 2 is moved in the focusing direction, indicated by arrow F in FIG. 1, that is in a direction parallel to the optical axis of the objective lens 1, as the elastic supporting members 3 are flexed elastically. With the bobbin 2 shifted in this manner, the objective lens 1 mounted on the bobbin 2 is moved in the same direction for focusing adjustment.
If the driving current derived from the tracking error signal is supplied from the driving control circuit via the electrically conductive elastic supporting member 3 to the tracking coil 9, there is produced a driving force which, in conjunction with the magnetic flux of the magnetic circuit section 4, operates to move the bobbin 2 in a direction normal to the optical axis of the objective lens 1. The bobbin 2 is moved in the tracking direction, indicated by arrow T in FIG. 1, that is in a direction normal to the optical axis of the objective lens 1, as the elastic supporting members 3 are flexed elastically. With the bobbin 2 thus shifted, the objective lens 1 mounted on the bobbin 2 is moved in the same-direction for tracking adjustment.
With the above-described objective lens driving device, the printed wiring board for relaying 10 is provided on each lateral side of the bobbin 2, and the coil ends 8a, 9a of the focusing coil 8 and the tracking coil 9 and one ends of the elastic supporting members 3 are connected to the printed wiring board 10. The driving current is supplied via the elastic supporting member 3 to the focusing coil 8 and the tracking coil 9. Instead of using the arrangement shown in FIGS. 1 and 2, the control current may also be supplied to the focusing coil 8 and the tracking coil 9 via a flexible printed circuit board 16.
With this objective lens driving device, one end of the flexible printed circuit board 18, connected to the upper surface of the bobbin 2, is connected to the upper end face of the bobbin 2, and the coil ends 8a, 9a of the focusing coil 8 and the tracking coil 9 are connected to a connection pattern 16a formed at an end of the printed circuit board 16, as shown in FIG. 3. By using the flexible printed circuit board 16, the flexible supporting member 3 need not be formed of an electrically conductive material, such that the flexible supporting members 3 may be formed of materials exhibiting desired properties, such as elasticity.
If the flexible printed circuit board 18 is used, there is no necessity of providing the printed wiring board for relaying 10 on the bobbin 2, so that the end 3a of the elastic supporting member 3 may be directly mounted via fitting supporting members 17 provided on both sides of the bobbin 2, as shown in FIG. 3.
Meanwhile, the focusing coil 8 and the tracking coil 9, employed in the above-described objective lens driving device, are formed by winding a linear material. The focusing coil 8 is formed by winding a sole linear material into a hollow prismatic shape and connection coil ends 8a are led out at upper and lower ends, as shown in FIG. 2. The tracking coil 9 is formed by winding a sole linear material for providing two rectangular-shaped coils 9b, 9c side-by-side and leading out connection coil ends 9a from one lateral sides of the coils 9b, 9c, as shown in FIG. 2. The tracking coil 9 is connected and unified to a lateral surface of the focusing coil 8 in the form of a hollow prism, as shown in FIG. 2. The focusing coil 8, thus carrying the tracking coil 9, is directly mounted on the bobbin 2 by connecting the side of the focusing coil opposite to its side connected to the tracking coil 9 attached to the inner wall of the through-hole 7, as shown in FIG. 7.
The coil ends 8a, 9a, drawn from the focusing coil 8 and the tracking coil 9, are electrically connected via an electrically conductive adhesive, such as a solder, to the printed wiring board for relaying 10 or to the flexible printed circuit board 16. Thus an operation of electrically connecting the coil ends 8a, 9a becomes necessary in the course of assembling the objective lens driving device, thus lowering the assembling efficiency.
During connection of the coil ends 8a, 9a, the coil ends 8a, 9a should not be slacked on the bobbin 2. If there is any slack on the coil ends 8a, 9a, the coil ends 8a, 9a tend to be inadvertently vibrated or shifted during movement of the objective lens 1 thus disabling the objective lens 1 to be correctly driven responsive to the control current.
In order for the objective lens 1 to be moved in a direction parallel to its optical axis and in a direction normal thereto with good response to the driving current supplied to the focusing coil 8 and the tracking coil 9, the center of gravity P of the bobbin 2 carrying the objective lens 1 as the movable part needs to be correctly in coincidence with the center of generation of the driving power for driving the bobbin 2.
In the above-described objective lens driving device, the driving force of driving the objective lens i in the focusing direction, that is in a direction parallel to the optical axis of the objective lens 1, is generated by the control current flowing in the coil portion 8b and a magnetic flux emanated by the magnet 14 from the upstanding piece 12 towards the opposite upstanding piece 13 so as to be interlinked with the coil portion 8b. The coil portion 8b is the portion of the focusing coil 8 wound in a direction normal to the optical axis of the objective lens 1. The driving force of driving the objective lens 1 in the tracking direction, that is in a direction normal to the optical axis of the objective lens 1, is generated by the control current flowing in linear 19a, 19b and a magnetic flux emanated by the magnet 14 from the upstanding piece 12 towards the opposite upstanding piece 13 so as to be interlinked with the linear portion 19b, 19b, as shown in FIGS. 1 and 4. These linear porions 19a, 19b are those portions of the coils 9b, 9c of the tracking coil 9 mounted on the lateral side of the focusing coil 8 inserted between the upstanding pieces 12, 13 extending parallel to the optical axis of the objective lens 1.
The coil portions 9b, 9c of the tracking coil 9 are connected so that the current will flow in the same direction in the linear portions 19a, 19b inserted into the upstanding pieces 12, 13.
For correctly generating the driving power for the objective lens 1 in a direction parallel to its optical axis, it is necessary for the coil portion 8b inserted between the upstanding pieces 12, 13 of the focusing coil 8 to be coincident with a line Y--Y' at the center of the magnetic gap defined between the upstanding pieces 12, 13, as shown in FIG. 4, while it is necessary for the control current flowing in the coil portion 8b to be interlinked highly correctly at right angles with the magnetic flux emanated into a space between the upstanding pieces 12, 13. For correctly generating the driving force in a direction normal to the optical axis, it is necessary for the center between the linear portions 19a, 19b of the rectangular-shaped coils 9b, 9c of the tracking coil 9 to be coincident with a line X--X' in FIG. 4 lying at the center in the width-wise direction of the upstanding pieces 12, 13, while it is necessary for the control current flowing in the linear portions 19a, 19b of the coil portions 9b, 9c to be interlinked with high precision with the magnetic flux radiated between the upstanding pieces 12, 13. For uniformly generating a driving force in each of the direction parallel to its optical axis and the direction normal thereto, it is necessary for the center of a line Z--Z' in FIG. 5 parallel to the optical axis of the objective lens 1, or the direction of the height of the coil portion 8b of the focusing coil 8 and the linear portions 19a, 19b of the tracking coil 9, to be coincident with the center of the height-wise direction of the magnet 14.
By arranging the focusing coil 8 and the tracking coil 9 with respect to the magnetic circuit section 4, the center of generation of a driving force generated by the control current supplied to the focusing coil 8 and the tracking coil 9 and the magnetic flux radiated from the magnet 14 for moving the objective lens 1 in a direction parallel to the optical axis and in a direction normal to the optical axis is positioned at a point of intersection of the line Y--Y' in FIG. 4, line X--X' in FIG. 4 and the line Z--Z' in FIG. 5.
The center of gravity P of the bobbin 2, as a movable part carrying the objective lens 1, is brought into coincidence with the point of intersection of the line Y--Y' in FIG. 4, line X--X' in FIG. 4 and the line Z--Z' in FIG. 5. The result is that the bobbin 2 may be moved with good response in a direction parallel to the optical axis of the objective lens 1 and in a direction normal to the optical axis of the objective lens 1 without producing any force of deflection, for example, distortion, against the driving force in the directions parallel and normal to the optical axis of the objective lens 1. Thus the objective lens 1, mounted on the bobbin 2, is responsive to the control current supplied to the focusing coil 8 and the tracking coil 9 to be moved correctly in the directions normal and parallel to the optical axis of the objective lens 1.
The objective lens 1 is mounted on the bobbin 2 so that its optical axis is located on the line X--X' in. FIG. 4 and parallel to the line Z--Z' in FIG. 5.
Meanwhile, the focusing coil 8 and the tracking coil 9, employed in the objective lens driving device, are of a three-dimensional configuration which is obtained by winding a linear material into a tube shape or a hollow prismatic shape, as explained previously. Thus it is extremely difficult for the focusing coils 8 and the tracking coils 9 used in the different objective driving devices to be of uniform constant size. If the focusing coil 8 and the tracking coil 9, thus differing in size, are mounted on the bobbin 2, the center of gravity P of the bobbin 2 inclusive of the objective lens 1 cannot be rendered constant. In particular, if the focusing coil 8 and the tracking coil 9, mounted at points spaced apart from the center of gravity P, suffer from variations in shape and size, the position of the center of gravity P of the bobbin 2 inclusive of the objective lens 1 is affected seriously.
Since the focusing coil 8, carrying the tracking coil 9 on its lateral surface, is directly bonded within the through-hole 7 in the bobbin 2, such as with an adhesive, it cannot be assembled in the bobbin 2 without difficulties, while the focusing coil also cannot be mounted at correct mounting position without difficulties. Consequently, the position of the center of gravity P of the bobbin 2 inclusive of the objective lens 1 differs from one objective lens driving device to another. That is, the center of gravity P cannot be set with high precision without difficulties.
The bobbin 2 is molded from synthetic resin, while the focusing coil 8 and the tracking coil 9 are formed by copper wires. The specific gravity of the synthetic resin of the bobbin 2 is approximately 1.5, while that of the copper wire of the focusing coil 8 and the tracking coil 9 is 8.9. Thus, if the focusing coil 8 and the tracking coil 9 undergo variations in shape and size and in mounting positions to the bobbin 2, it becomes impossible to accurately set the position of the center of gravity P of the bobbin 2 as a movable part inclusive of the objective lens 1 from one objective lens driving device to another.
If the position of the center of gravity P of the movable part is not constant from one objective lens driving device to another, the center of generation of the driving power generated by the interaction between the focusing coil 8 and the tracking coil 9 on one hand and the magnetic circuit section 4 on the other hand is not coincident with the center of gravity P of the movable part, such that the objective lens 1 cannot be moved in directions parallel and normal to its optical axis without producing the force of deviation, such as distortion. The result is that the light beam radiated on the signal recording surface of the optical disc cannot be controlled precisely as to focusing and tracking so that information signals cannot be recorded/reproduced with good recording and/or reproducing characteristics.
The objective lens driving device is arranged so that the coil portion 8b on one side of the focusing coil 8 wound in the shape of a hollow prism is inserted into a space defined between the upstanding pieces 12, 13 making up the yoke 5, as shown in FIGS. 1 and 8. Thus the coil portion 8c on the opposite side of the coil portion 8b of the focusing coil 8 also faces the upstanding piece 12 carrying the magnet 14. If the control current flows through the focusing coil 8, not only is a driving force f.sub.1 produced by the interlinkage between the effective magnetic flux Bg radiated into a space between the upstanding pieces 12, 13 and the coil portion 8b, but a driving force f.sub.2 is generated by the interlinkage between the stray magnetic flux Bg' radiated from the magnet 14 towards the rear side of the upstanding piece 12 and the coil portion 8b. The driving force f.sub.2, generated by the interaction with the coil portion 8b interlinked with the stray magnetic flux Bg', is a force opposite to the driving force f.sub.1 generated by the interaction of the opposite side coil portion 8b interlinked with the stray magnetic flux Bg, and operates for canceling the driving force of driving the objective lens 1 along the optical axis. Thus it becomes impossible to exploit the driving force of driving the objective lens 1 along the optical axis.
With the objective lens driving device, shielding means, such as a shielding plate, is provided for shielding the stray magnetic flux for eliminating the stray magnetic flux. Alternatively, the focusing coil 8 is increased in size. If such shielding means are provided or the size of the focusing coil 8 is increased, the objective lens driving device itself is increased in size.
The portion of the focusing coil 8 of the objective lens driving device operating in conjunction with the magnetic flux emanated from the magnet 14 for generating a driving force of driving the objective lens 1 along the optical axis is only a fractional portion, shown shaded in FIG. 7, that is a portion of the coil portion 8b which is inserted into the space between the upstanding pieces 12, 13 and which faces the magnet 14. The portion of the tracking coil 9 of the objective lens driving device operating in conjunction with the magnetic flux emanated from the magnet 14 for generating a driving force of driving the objective lens 1 in a direction normal to the optical axis is only a fractional portion, shown shaded in FIG. 8, that is a part of the linear portions 19a, 19b of the coil portions 9b, 9c which faces the magnet 14. That is, the portions of the focusing coil 8 and the tracking coil 9 operating for generating the driving force of driving the objective lens 1 account for only one quarter of the entire portions of the coils 8 and 9, that is the utilization efficiency of the coils 8, 9 is extremely low. Since the utilization efficiency of the focusing coil 8 and the tracking coil 9 is low, the current required for driving the objective lens 1 increased, so that heat evolution from the objective lens driving device is also increased. This heat evolution tends to affect the operation of the semiconductor laser as a light source constituting the optical pickup device, thus obstructing stable light beam excitation.
Since the focusing coil 8 employed in the objective lens driving device is wound in the shape of a hollow prism shape, its self-inductance tends to be increased. Besides, since the upstanding piece 12 of the yoke 5 constituting the magnetic circuit section 4 is adapted for being inserted into the focusing coil 8 in the shape of the hollow prism, the upstanding piece 12 operates as an iron core for further increasing the self-inductance of the focusing coil 8. With the self-inductance of the focusing coil 8 increased in this manner, phase rotation is rapidly increased beyond 180.degree. in a high frequency range of the focusing error signals if the driving current corresponding to the focusing error signal is supplied to the focusing coil 8 via the driving control circuit for moving the objective lens 1, such that focusing control responsive to the focusing error signals cannot be achieved. For enabling the focusing control, the control circuit for detecting the focusing error signal for supplying the control current to the focusing coil 8 is designed for effecting electrical phase correction. If the amount of this phase correction is increased, the harmonics of the driving current supplied to the focusing coil 8 is increased in proportion to the amount of phase correction, thus increasing power consumption. If the power consumption is increased, the operation of the semiconductor laser constituting the optical pickup device becomes unstable due to heat evolution.